FIGS. 2(a) to 2(f) are cross sections illustrating process steps in a method of producing a T-shaped gate electrode of a prior art semiconductor device. In the figures, reference numeral 1 designates a compound semiconductor substrate, numeral 1a designates a recess, numeral 2 designates an insulating film, numeral 6 designates a gate metal, numeral 6a designates a T-shaped gate electrode, numeral 7 designates a photoresist film sensitive to electron beam exposure, numeral 8 designates a photoresist film sensitive to light exposure and numerals 7a and 8a designate apertures.
A description is given of process steps of a T-shaped gate electrode with reference to FIGS. 2(a) to 2(f).
First, as illustrated in FIG. 2(a), a photoresist 7 for electron beam exposure has deposited on it a photoresist 8 for optical exposure. The photoresist 7 is disposed on a compound semiconductor substrate 1. During the deposition, the photoresist 7 for electron beam exposure and the photoresist 8 for optical exposure must not mix with each other, namely, the electron beam photoresist resin must not be dissolved by the solvent of the optical photoresist 8. Therefore, photoresists comprising resin and solvents satisfying the above-described condition, are selected as the electron beam photoresist and the optical photoresist. Secondly, by a predetermined exposure of the photoresist film 8 in an optical exposure apparatus and development of the film in a prescribed developer, as illustrated in FIG. 2(b), a first aperture 8a of a relatively large width is formed in the optical photoresist film 8. Thirdly, by electron beam exposure of a prescribed portion of the electron beam photoresist film 7 in an electron beam exposure apparatus through this first aperture 8a, and development of the film in a prescribed developer, as illustrated in FIG. 2(c), a second aperture 7a of a relatively small width is formed. Next, as illustrated in FIG. 2(d), using the photoresist films 8 and 7 in which the first aperture 8a and the second aperture 7a are respectively formed as a mask, a recess 1a is formed by etching the compound semiconductor substrate 1. Then, as illustrated in FIG. 2(e), a gate metal 6 is deposited on the whole surface of the substrate and lift off is conducted to form a T-shaped gate electrode 6a as illustrated in FIG. 2(f).
As described above, in the prior art method of forming a T-shaped gate electrode, by electron beam exposure of the electron beam photoresist film 7 and development of the film, the photoresist aperture pattern for prescribing the lower part electrode width of the T-shape gate electrode (gate length) is produced. In this pattern exposure method employing electron beam irradiation, in drawing patterns, it is difficult to enhance throughput in manufacturing semiconductor devices.
Further, in the prior art process, in order to produce the T-shaped gate electrode with improved precision and stability, when the photoresist aperture pattern for prescribing the upper part electrode width of the T-shaped gate electrode is produced, namely, when the optical photoresist film 8 is developed, it is necessary that the electron beam photoresist film 7 below the photoresist film 8 not be developed by the developer of the film 8. In addition, as described above, when the photoresist film 8 is deposited on the photoresist film 7, these films are required not to mix with each other. Therefore, the degree of freedom in selecting photoresist materials is unfavorably restricted to a great extent.
In addition, in the pattern exposure method with electron beam irradiation, i.e., direct drawing method, at present, the resolution limit is at most 0.2 to 0.25 microns and the gate length is not shortened below that limit.